This research proposal is concerned with the molecular mechanisms involved in the control of virus gene expression at transcriptional, post-transcriptional and translational levels. We proceed with the bacteriophage T7 - Escherichia coli system as a simple virus-host model system and aim to obtain a better and thorough understanding of virus development which is applicable to other bacteriophages and animal viruses. T7 genes and corresponding mRNAs and proteins have been subject to an extensive genetic and biochemical analysis and it is now well-established that the early genes are transcribed by the host RNA polymerase and the late genes are transcribed by the T7-coded RNA polymerase which is one of the early gene products. However, this unique transcriptional control by two different RNA polymerases is not sufficient to explain the early-late switch in T7 development. Our specific research aims are directed towards the following areas: 1. T7 early mRNA is chemically stable but functionally unstable. Unique structural features of the RNA will be investigated. 2. Possible translational discrimination between T7 early mRNA and late mRNA in T7-infected cells will be investigated with a special attention to the F factor-mediated restriction of T7 in F positive f E. coli. 3. "Host shut-off" function of T7 which inactivates the E. coli RNA polymerase will be elucidated. Isolation, purification and characterization of an inhibitor protein of E. coli RNA polymerase will be performed together with a genetic approach to determine the gene of T7 which codes for the inhibitor protein.